1. Field of the Invention
The present invention relates to a method for forming a redistribution layer in a wafer structure, particularly to a method for embedding redistribution layer in the passivation layer of the wafer structure.
2. Description of the Related Art
FIG. 1 shows a cross sectional view of a conventional wafer structure. The conventional wafer structure 10 comprises a wafer 11, a plurality of bonding pads 12, a first passivation layer 13, a redistribution layer 14, a second passivation layer 15, an under bump metallurgy layer 16 and a solder bump 17.
The bonding pads 12 are disposed on a surface of the wafer 11, and the material of the bonding pads 12 is usually aluminum, copper or the like. The first passivation layer 13 covers the wafer 11 and part of each bonding pads 12 so as to expose a conductive surface on each bonding pads 12. That is, the first passivation layer 13 does not cover the top surfaces of the bonding pads 12 completely. The material of the first passivation layer 13 is usually benzocyclobutene (BCB), polyimide (PI) or the like. The redistribution layer 14 is disposed over the first passivation layer 13 and is used for electrically connecting the bonding pads 12 and the under bump metallurgy layer 16. The material of the redistribution layer 14 is usually aluminum or the like. The second passivation layer 15 is disposed over the first passivation layer 13 and is used for protecting the redistribution layer 14. The material of the second passivation layer 15 is usually benzocyclobutene (BCB), polyimide (PI) or the like. The under bump metallurgy layer 16 is disposed on a predetermined location and has the solder bump 17 thereon. The under bump metallurgy layer 16 includes an adhesion layer, a barrier layer and a wetting layer (not shown), and is used for enhancing the attachment between the solder bump 17 and the redistribution layer 14. The material of the solder bump is usually tin/lead alloy.
FIGS. 2a to 2m show a conventional method for forming a redistribution layer in the wafer structure 10 of FIG. 1. The conventional method is described as follows. First, a wafer 11 having a plurality of bonding pads 12 is provided, as shown in FIG. 2a. A first passivation layer 13 is then formed on the wafer 11 to protect the wafer 11 by coating or deposition. The first passivation layer 13 does not cover the top surfaces of the bonding pads 12 completely but exposes the conductive surface 121 of the bonding pads 12, as shown in FIG. 2b. Then, a conductive layer 14a is formed over the first passivation layer 13 and the bonding pads 12 by sputtering, as shown in FIG. 2c. The conductive layer 14a is patterned according to the following steps. A photoresist film is applied to the conductive layer 14a and is exposed and developed to form a patterned photoresist film that serves as a mask. Then, the conductive layer 14a is selectively removed by wet etching to form the redistribution layer 14, as shown in FIG. 2d. Then, the second passivation layer 15 is formed by coating or deposition on the redistribution layer 14 to protect the redistribution layer 14, as shown in FIG. 2e. The second passivation layer 15 is selectively removed by utilizing exposing and development technique so as to have a plurality of openings 151 and expose part of the redistribution layer 14, as shown in FIG. 2f. Then, a conductive layer 16a is formed over the second passivation layer 15 by sputtering, as shown in FIG. 2g. 
A photoresist film, for example, a dry film or a liquid photo resist layer, is applied to the conductive layer 16a. By an appropriate way, for example, patterning, a plurality of openings are defined on the photoresist film. The photoresist film is selectively removed so that a patterned photoresist film 18 remains on the opening 151, as shown in FIG. 2h. Then, the conductive layer 16a is patterned by etching according to the mask of patterned photoresist film 18. For example, part of the conductive layer 16a is removed by wet etching, and the conductive layer 16a on the opening 151 remains to form the under bump metallurgy layer 16, then the patterned photoresist film 18 is stripped, as shown in FIG. 2i. 
Referring to FIG. 2j, the entire surface is covered by another photoresist film 181 on which an opening 182 corresponding to the solder bump 17 is formed by patterning. Then, a silver paste 19 is filled in the opening 182 by screen printing, as shown in FIG. 2k. The solder bump 17 is made of the silver paste 19 after reflow, as shown in FIG. 2l. Finally, the conventional wafer structure 10 is formed after the photoresist film 181 is stripped, as shown in FIG. 2m. 
A shortcoming of the conventional wafer structure 10 is that the redistribution layer 14 is on the surface of the first passivation layer 13, which will cause delamination easily. That is, the redistribution layer 14 cannot be fixed on the first passivation layer 13 tightly due to the poor attachment therebetween. Thus, the performance of the redistribution layer 14 will be reduced, and moreover, the packaging may fail.
Consequently, there is an existing need for a novel and improved method for forming a redistribution layer in a wafer structure to solve the above-mentioned problem.